The Russian government’s cyber-espionage campaign against the American political system began more than a year ago and has been far more extensive than publicly disclosed, targeting hundreds of key people — Republicans and Democrats alike — whose work is considered strategically important to the Putin regime, official sources told NBC News.

The targets over the past two years have included a Who’s Who of Hillary Clinton associates from her State Department tenure, the Clinton Foundation and her presidential campaign, as well as top Republicans and staffers for Republican candidates for president.

Starting in earnest in 2015, Russian hackers used sophisticated “spearphishing” techniques to steal emails and other data from Capitol Hill staffers, operatives of political campaigns and party organizations, and other people involved in the election and foreign policy. That’s according to NBC News interviews with more than two dozen current and former U.S. officials, private sector cybersecurity experts and others familiar with the FBI-led investigation into the hacks.

“For the past two years, there has been a massive increase in hacking by the Russians,” said Dmitri Alperovitch, a cybersecurity expert whose CrowdStrike firm was retained to investigate the hack of the Democratic National Committee.